Mode filter



March 24, 1953 W. M. PRESTON MODE FILTER Filed Sept. 18, 1945 .|NVENTOR W|LL|AM M. PRESTON ATTORN EY Patented Mar. 24, 1953 UNITED STATES.

MODE FILTER-q.

Navy

Application September 18, 1 945, SerialNm 617,134

11 Claims; 19 The. present invention. relates; to electrical energy,,andmore particularly. tothe transmission ofsuch energy from one. place to. another by means'ofhollow pipes or thelike. To a. greater degree of particularity, the invention. isv concernedwith devices for eliminatingfrom a hol low, wave guide one mode of 1' transmission. that. may beundesirable while simultaneously permitting, passage of a desired mode. The subject matter of this invention is disclosedin the copending applicationof'shepard Roberts, S. N. 510,990, filed November 19,1943, PatentNo. 2,513,205, issuedJune 27; i950, andin. applicants copending, application, Serial no. 610350, filed August, 11, 19%,, now Patent. No. 2,584,399; issued February 5', 1952; wherein, are? described rotatable jointsfor'fwave guides. Ad'- vantage is therein taken of the fact that in a cylindrical wave guide carrying energy in a mode ofiaxial' symmetry such as the 'IMnor the TEm mode, the wave guide may-be made of two coaxial "abuttingsections and one section rotated on. the common axis with. respect to the other without disturbingtthe passage of the energy.

Itshould be noted that the terminology byv whichv modes of" oscillation of electrical energy in wave guides may be designated isoftwosorts. A wave designated as TMdi may also; by another. system. be calledfEti, or ina cylindrical wave guide simply E0, while a Wavedesignated" asTEn mayj'be callediin the other system, Hi1, or in acylindrical wave guide" simply Hi. In the present application, the system in which. the mode is cl'esignatedby the name of thev component lying wholly transverse to the axisof' the Wave: guide will'be employed. Thus waves" will herein be designated as transverse magnetic" (TM) or transverse electric (TE).

In using sections of cylindrical wave guide to provide arotatable'joint for a waveguide sys--- tem, it is often and usually necessary" to 'couple' energy into and out of' the cylindrical sections from and into other types of transmission lines; Thus; forexainp'le, in' electrical system that uses'a rectangular wave guide as a" transmission line, the" insertion-of a rotatable joint in the transmission line requires that energy" of the mode being" used in the rectangular" wave guide" be converted to an axially symmetrical mode-upon entering the'rotatable joint and then be recon--- verted to the mode of the rectangular wave guide upon leaving the rotatabljoint. However, it is difiicult and very often impossibleto set up initiallyin a wave guidea singlepure mode of oscillation. Evenafter choosing the dimensions of'-the'-various wave guide sections so' as to pro-'-- vide the most favorable combination for'transferring energy with minimum losses,v it is usuallyfo'und'that a. substantial amount of energy is being lost in standingwaves caused byoneor more unwantedmodes of oscillation; In a'rotatable joint, the unwanted mode often takes the? form of one-that is not axially symmetrical. The. presence of sucha mode ofoscillation results in.

losses that vary asthe rotatable joint is rotated,

causing varying attenuation in signal strength that reduces the efficiency, of the complete. systern. Furtherdifiiculties are introduced by'such an axially non-symmetrical mode. in that it'results in the rotatable joint beingvfrequency, sensitive andof relatively narrow bandwidth, thus' requiring careful tuning operations. In my'invention I'have provided a mode filter for wave guides that filters outa mode of oscillation that is non-symmetrical axially while simultaneously having'no eif'ect 'onan axially symmetrical mode of oscillation.

It is an object ofmy invention to provide'such a filter thatwill respond to and effect theattenuation of axiallynon symmetrical modes'of' oscillation" while simultaneously being non-responsivefto an axially 'symmetrical'mode of oscillation;

It is another object of my invention toprovide for such afilter a-supporting structure that will not attenuate the'desired' modeof oscillation.

Itis'afurther' objectof my invention to provide ings; the figures of which illustrate typical em'-- b'odimentspf the invention.

In the*drawings:

Fig; 1 illustrates a portion of a waveguide transmission line system having a rotatable joint therein;

Fig: Zds-a cross section of a portion of a rotat able joint for a-Wave guide, broken away at one end; showing a filter in accordance with my in-- vention in place 1 therein;

Fig. 3 is' 'a cross section taken on line-III--III of Fig. 2 showing the'structureof a filter of my invention; and

Fig; 4 is across-section ofa; portion of arotat--- lar wave guides.

3 able joint showing an alternative embodiment of my invention.

In Fig. 1, a transmission line of the rectangular wave guide form I is connected to a section of cylindrical wave guide 2, which is part of a rotatable joint. Electrical coupling and anti-friction devices are indicated by the housing 3 which encloses them. The housing 3 is attached to a second section of cylindrical wave guide 4, which is coaxial with the first section 2. The second section 4 is rotatable with respect to the first section 2 about their common axis.

Typical structure of the sort enclosed in the housing 3 is shown in the housing 3' in Fig. 4, and discussed in detail in the hereinabove referred to patent and copending application. The

two sections of coaxial line 2 and A are coaxially aligned, and abutting but not touching each other at their adjacent faces and 6. The housing 3' is permanently attached to one cylindrical wave guide section 4, and has an annular slot 1 provided therein for the purpose of facilitating electrical coupling between the two sections 2 and l. Anti-friction bearings 8 in concentric races 25 and 26 are provided to permit easy rotation of one cylindrical section 2' with respect to the other 4. The outer race 25 is fixedly held in the housing 3 by the threaded retainer ring 21. The face ring 5 is affixed to the inner race 26 on one side thereof and the threaded ring I5 is afiixed to the same race 26 on the opposite side thereof. A second threaded ring It is afiixed to the other cylindrical wave guide section 2, and is adapted to cooperate rotatably with the first threaded ring [5. Rotation of the cylindrical section 2' and its thereunto afiixed ring 16 with respect to the cooperating ring l5 efiects axial motion of the cylindrical section 2' with respect to the abutting section 4', while simultaneously preserving constant and undisturbed the spacing between the abutting faces 5 and 6. This spacing is important, similarly to the slot 1 in coupling energy from one cylindrical section 4 to the other '2'. The cylindrical wave guide sections .2 and 4 of Fig. 1 are spaced and supported in a fashion similar to the sections 2 and 4 of Fig. 4.

Returning now to Fig. 1, the second section 4 is connected to a rectangular wave guide 9 of the same sort as the original transmission line I. The two parts of the rectangular wave guide i and 9 are thus movable, one with relation to the other about the axis of the cylindrical wave guide sections 2 and 4 because of the presence of the rotatable joint.

As normally and preferably operated, energy is carried in the rectangular wave guides or E, in the TEo1 mode. Upon entering the cylindrical wave guide 2 from the transmission line i this 'IEoi mode energy is changed to energy in the TM01 mode for cylindrical wave guides, which is an axially symmetrical mode. The TMm mode is easily excited in the cylindrical section 2 by energy in the TEM mode emanating from the transmission line I. Upon leaving the cylindrical section 4 and entering the rectangular section 9, the energy in the TIVI01 mode is reconverted to energy in the TEM mode for rectangu- Electrical energy may pass in either direction through the system of Fig. 1.

As hereinabove explained, a single pure mode of oscillation is not to be expected initially in the cylindrical wave guide sections 2 and 4. Even after all known precautions as to the dimensions of the various wave guide sections and to matching impedances have been taken, a mode of oscillation that is non-symmetrical about the common axis of the cylindrical sections 2 and 4 will be present. This may be and often is the TEn mode, and it is undesirable for the reason that it consumes energy that would otherwise pass through the rotating joint in the desired mode, TMOl. Energy consumed in the undesired TEu mode of oscillation represents a corresponding attenuation of the desired signal passing through the rotating joint.

Since the TE11 mode of oscillation is non-symmetrical about the axis of the cylindrical wave guide sections 2 and 4, the energy it consumes will vary as the two sections 2 and i of the rotating joint are rotated with respect to each other,

thereby causing variable attenuation of the signal sought to be transmitted through the wave guide system of Fig. 1. Where for example the wave guide system of Fig. l is being used in a radio echo detection system to carry energy between a transmitter-receiver combination and a rotatable scanning antenna, both the outgoing and the incoming signals will be subject to such variable attenuation, with the result that the information presented by the detection system will be unstable and perhaps useless. Thus it is seen that in a rotating joint like that shown in Fig. 1 the TEn mode of oscillation in the cylindrical sections 2 and 4 of the joint is undesirable and should be eliminated.

I have found that a ring of conducting material lfi shown particularly in Figs. 2 and 3 mount ted transversely in a section of cylindrical wave guide 2, as illustrated in Fig. 3, will be energized by and reflect energy in the T5111 mode of oscillation, but will not be energized by energy in the TM01 mode. The TMm mode of oscillation will not be affected by the ring I0 since the electric vector for the TM01 mode of oscillation is everywhere perpendicular to the circumference of the ring H3. The electric vector for the T1311 mode is, however, not everywhere perpendicular to the circumference of the ring, and there will consequently be currents induced in the ring In, and the ring I6 will oscillate at the frequency of the energy exciting the ring it]. I have found further that if the mean circumference of the ring it be approximately equal to 1.2 times the free space wave length corresponding to the frequency of the wave being carried in the wave guide 2, the ring it will be resonant to that frequency. Such a ring lfl positioned in the wave guide 2 will cause substantially total reflection of oscillations in the TEn mode in the wave guide 2. A ring I!) of this sort will operate as a filter to inhibit or substantially prevent the propagation of the TEn mode of oscillation in the wave guide 2 when further positioned as illustrated in Figs. 2 and 4.

In Fig. 2 there is shown a cross section of the lower end of the apparatus illustrated in Fig. 1 comprising parts of the section of rectangular wave guide I and the section of cylindrical wave guidev 2. In this form of apparatus both the terminating wall it and the reflecting filter ring i 9 are fixed in position with respect to the junction of the rectangular wave guide I and the cylindrical wave guide 2. The metallic ring it, preferably positioned concentric with the wave guide 2, is mounted upon a hollow cylinder I2 of an insulating material, preferably polystyrene. The cylinder l2 may be mounted in any convenient manner on the wall H and in Fig. 2 it is shown fitted about a slight cylindrical projection I 3 the edges of which I serve to positionathe It is preferably" cylinder I2 9 Thecylinder cemented to'thewall I I': and its-projection- I3. The advantage oftheaxial type ofmount- I2 of the reflecting ring I9 shown in Fig. 2.? is that" the insulating material of the support IZ- for the filter ring it isarranged in a manneradapted 'to absorb a minimum ofenerg-y from the fieldi of" the TMIll mode ofoscillation in the cylindri'cal wave guide 2,; the electric vector of said field being radial with respect to-the cylindrical' wave guide 2. It is to I be I understood that an adjust able= support for the-ring; I9 could be-provided;

if" desired; by a suitable arrangement for mov ing the cylinder- I2 axially with: respect to the wave guide- 2".

the endshown in Fig.- 2. It istobe-understood that'the apparatus of Fig; Z' isinitself'a com-- plete system for transforming electrical energy able-to function in fromone mode to another; both-directions;

The ring I is so positioned witlrrelation to the rectangular waveguide I-that oscillations in the-TEn mode set up at the junction of the wave guides I and 2 whicharereflected by the ring; Ill will cancel the-'TEir mode oscillations proceeding' from said junction away; from the ring IIJ, thereby inhibiting or even preventing-the propagation of the TEn mode of" oscillation down the wave guide. 2: For such action the plane of the filter" ring; ID should be approximately; an electrical "quarter wave lengthfor the TEn mode away from the aforesaid junction, as" shown in Fig. .2, or any odd-number'of'quarter wavelengths away from-said junction. The-line III III*lies.

in" the plane of the ring; Ill; and" the. line B' B" represents the centerline of the rectangular wave guide I, and*hencein effect themid-poiht of'the.

junction of the rectangular wave guide I and the cylindrical wave guide 2; The distance between these two lines, III-"IIIand'B;-'B should be approximately an electrical. quarter wave length but may be, any odd number of electrical'jquarterwave.

lengths as hereinabovespecified... Thisisinac:

cordance with well known. principles. of I wave.

transmission.

' In acompleterotatable joint asshown in Fig 1, th'e distance between the filterrin s .I I! in each.

endof the. apparatus ismadesuch asto avoid resonance for the TEu modeof oscillation. Each filter ring Ill. should be positioned, for cancellation. of. the T511 mode. oscillations proceeding down. its cylindrical wave. guide section.2.or 4 ashereinabove taught, and. the. length. of the cylindricalwave guide system shouldbe such as, to. obtain the non-resonance. spacingbetween filter. rings Ill. Apparatus, such as that contained inwthe housing 3', shown inFig, 4, comprising.

threaded collars I5 and I6, willpermit; onecylindricaljwave. guidei sectionyl to... be. moved. axially with respect. to the: other cylindrical section; 4. without varying the spacing between-the abutting faces. -.of.the twocylindrical sections, asldescribed hereinabove.

Because of. the presence of .the filtenrin I and art-identical companion ring= inithe cY1in d-ri al w r uide; se tions 2.. and;- 4. the. distance: between; the; closure I I. and, the corresponding, losure I1 at the otherendof thecylindri al wave. guide .system is no longer sensitive: to TEn .modeg; resonances... As. a; result, therrotating joint is longer-.,sharpm sensitive-to frequency. changes,:.

The upper end oi the rotatable joint-of Fig. 1-, comprisingithe: companion cy-lindrical wave guidesection 4' and therectangular wave guide 9 connected thereto; isidentical to of'the-closures II and II.

the cylindrical wave guide 2. This preferred spacing is indicated in Fig. 2, where a spacing" of one-half wave length is shown. With this preferredspacing. the closure II will'reflect waves= in-thelMtr mode-so-that they will reenforce similar 'IMdr mode= oscillations proceeding down the cylindrical wave guide 2 from the junction-of the two'wave' guides I and 2, in' accordance withthe well known principles of wave transmission. 05+

cillations in the TEu mode do not reach" the closure I I; having been reflected away from the closure II by the reasonant filter ring I0 Similar spacing of the closure ll of the other endl of the cylindricalwave-guide system from'the corre-- sponding junction of two wave guides 4 and 9 is provided.

If desired, the filter ring- IEI and the closure may either or both be mademovable along thelongitudinal axis of the cylindrical wave guide system. A modification of the apparatusofFig; 2 including these featuresis=shown in Fig: 4. A

part of the cylindrical'wave guidestructureoftherotatingjoint of Fig. 1 isshown includingthecy lindrical wave guide section' l" and the housing The closure I'I of Fig. 1- is-altered and thecylindrical section 4' is extended in-length to provide room for amovable-closure I8. The closure- I8- may be axially'adjusted within the cylindricalwave guide-thy meansof a screw I9 and a hand nut 29. The hand nut 29 isrotatably held against motion along the longitudinal axis of the'wave' guide 4 by the wall 2 I, which is not theelectrical closure for the cylindrical waveguide 4':

surface of the wave guide 4" by *meansof springe like metallic-fingersrfi, separatedby'slots 23. A

resonantfilter ringl 0', which may be identicalto" the'ring II) of Fig. 2 is'mounted' transversely with respectto the wave guide duponan insulating support 24 which is adapted to positionthe filter ring III concentrically withinthe. wave guider4". The; supporting insulator 24 may conveniently; be made of mica or mica-bearingmaterial. The position' ofthe filter ring; I0 may be adjustedby sliding the insulatingstructure 2'4" axially along the wave guide 4.

As in the apparatus of Fi 2', the apparatus of Fig. l is preferablyadjusted so that the filter. ring, Iflis spacedfrom the junction of the cylindrical waveguide 4 and therectangular wave guide 9..

approximately an electrical quarter-wave length for the TEn mode of oscillation. Similarljy,.,the..

closure I8 is preferably spacedif'romrthe same junction a distance equal'approximately to an electrical'half wave length for the TMM mode. of oscillation. These dimensions are indicated on Fig. 4; As in the structure ofFig, 2, a complete.

rotatingjoint of the .Fig. 4 embodiment has identical structureat the opposite ends. The same refinements and adjustments that apply to the.

apparatus of Fig. 2' apply to the. apparatus of Fig. 4. were the former when properly adjusted.

If desired, a rotating joint may have the apparatus, of Fig. 4 in one end, as in section G5,. andthe. apparatus of Fig. v2, .in the; other end as.

The" closure I8 makes electrical contact with the inner Indeed, the. latter functions as if. it-

aesaaoc:

7 section 2thereof. 'Further, the fixed closure'l I of Fig. 2 may be used if desired, with the movable ring supporting structure 24, of Fig. 4, or other suitable movable structure.

It is to be understood that electrical energy may pass through a rotating joint having the filter of my invention therein in either direction. The filter rings In and ill will operate to inhibit energy in the TE11 mode from entering as well as leaving a connected rectangular wave guide I or 9.

Although I have shown and described only certain specific embodiments of my invention, I-am fully aware of the many modifications possible thereof. Therefore this invention is not to be limited except insofar as is necessitated by the prior art and the spirit of the appended claims.

Iclaim:

1. In a cylindrical wave guide fed at a point in a side thereof by a generator that introduces into said guide electric waves in the TM01 and the 'I'En modes, means for attenuating the TEn .mode but not the TMm mode comprising a metallic annulus positioned within said guide in aplane substantially transverse to the axis thereof, said plane being spaced from said point by a distance substantially equal to one quarter of the length of one of said waves in the TEn mode. 2. In a cylindrical wave guide system fed at a point in its side by a generator of energy of a certain frequency in the TM01 and the Thin modes, a metallic annulus having its circumference substantially equal to 1.2 times the freespace wave length of said energy positioned axially within said wave guide in a lane substantially transverse to the axis thereof for resonantly responding to and reflecting waves of said energy in the 'IEn mode, said plane being substantially one quarter of a TEn mode wave length from said point, whereby said reflected waves meet waves of the TEn mode at said generator in substantially opposing phase to effect mutual cancellation of waves in said TE11 mode.

3. A mode transformer for transferring electromagnetic energy in the T1301 mode in a rectangular wave guide to electromagnetic energy in the TM01 mode in a cylindrical wave guide comprising, a, right-angle junction between a rectangular wave guide and a cylindrical wave guide wherein the broad walls of said rectangular wave guide are perpendicular to the longitudinal axis of said cyl'mdrical wave guide whereby os cillations principally in the TM01 mode and incidentally in the TEn mode are excited in said cylindrical wave guide when said rectangular wave guide is energized in the TEOI mode, and means for selectively attenuating the TEn mode comprising a conducting end closure for said cylindrical wave guide spaced from the central axis of said rectangular wave guide a distance substantially equal to a half wave length of energy in the TMOl mode, and a conducting annulus positioned coaxially within said cylindrical wave guide and spaced from the central axis of said rectangular wave guide in the direction of said end closure a distance substantially equal to a quarter wave length of energy in the TEn mode. 4. Apparatus in accordance with claim 3 wherein said conducting annulus is maintained in position by an insulating annulus surrounding said conducting annulus and contacting the inner surface of said cylindrical wave guide.

5. Apparatus in accordance with claim 3 wherein the mean circumference of said conducting annulus is substantially 1.2 times the free space wave length of said energy, and said conducting annulus is positioned by an annulus of insulating material surrounding said conducting annulus and contacting the inner surface of said cylindrical wave guide.

6. A mode transformer for transferring electromagnetic energy in the TEM mode in a rectangular wave guide to electromagnetic energy in the TMOl mode in a cylindrical wave guide comprising, a right-angle junction between a rectangular wave guide and a cylindrical wave guide wherein the broad walls of said rectangular wave guide are perpendicular to the longitudinal axis of said cylindrical wave guide whereby oscillations principally in the TM01 mode and incidentally in the Thin mode are excited in said cylindrical wave guide when said rectangular wave guide is energized in the TEM mode, and means for selectively attenuating energy in the 'IEn mode comprising a metallic end plate positioned transversely of said cylindrical wave guide and spaced from the central axis of said rectangular wave guide a distance substantially equal to a half wave length of energy in the TM01 mode, and a resonant ring supported coaxially Within said cylindrical wave guide by an insulating cylinder, said cylinder being secured to said end plate and having a length to position said ring from the central axis of said rectangular wave guide a distance in the direction of said end plate substantially equal to a quarter wave length of energy in the 'IEn mode.

'7. Apparatus in accordance with claim 4 wherein the mean circumference of said ring is substantially 1.2 times the free space wave length of said energy.

8. A mode transformer for transferring electromagnetic energy in the TEOl mode for rectangular wave guides to electromagnetic energy in an axially symmetrical mode for cylindrical wave guides comprising, a right-angle junction between a rectangular wave guide and a cylindrical wave guide wherein the broad Walls of said rectangular wave guide are perpendicular to the longitudinal axis of said cylindrical wave guide whereby oscillations in axially symmetrical and axially unsymmetrical modes are excited in said cylindrical wave guide when said rectangular wave guide is energized in the TEO]. mode, and means for selectively attenuating energy in the axially unsymmetrical mode comprising a conducting end closure for said cylindrical wave guide spaced from the central axis of said rectangular wave guide a distance substantially equal to a half wave length of energy in the axially symmetrical mode, and a conducting ring dimensioned to be resonant to said energy and positioned transversely and coaxially of said cylindrical guide and spaced from the central axis of said rectangular wave guide in the direction of said end closure a distance substantially equal to a quarter wave length of energy in said axially unsymmetrical mode.

9. In combination with a, right-angle junction between a rectangular wave guide and a cylindrical wave guide wherein the broad walls of said rectangular guide are perpendicular to the longitudinal axis of said cylindrical wave guide, means for improving the transfer of electromagnetic energy in the TEOI mode from said rectangular wave guide to electromagnetic energy in the TM01 mode in said cylindrical guide comprising, a conducting closure for said cylindrical guide spaced from the central axis of said rectangular guide a distance substantially equal to a half wave length of energy in said TMOl mode.

9 and a conducting ring positioned coaxially within said cylindrical guide, said ring being disposed in a plane transverse to the longitudinal axis of said cylindrical wave guide and spaced from said central axis in the direction of said closure a distance substantially equal to a quarter wave length of energy in the TE11 mode.

10. In a wave guide system, a right-angle junction of a rectangular wave guide and a circular wave guide, said junction being adapted to excite oscillations in said circular guide in the TM01 and TEu modes when said rectangular guide is energized in the TEM mode, a conducting end closure adjustably positioned in said circular guide at a distance from the central axis of said rectangular guide substantially equal to an integral number of half wave lengths of energy in the TMM mode, and a metallic annulus having a mean circumference equal to 1.2 times the free-space wave length of said energy adjustably positioned coaxially within said circular guide in a plane transverse to the longitudinal axis of said circular guide, said plane being spaced from the central axis of said rectangular wave guide in the direction of said end closure a distance substantially equal to an odd number of quarter wave lengths of energy in the T1311 mode, said combination being adapted to attenuate energy in the TE11 mode and to propagate energy in the TMM mode without attenuation.

11. Apparatus of the class described comprising in combination a right-angle junction of a cylindrical wave guide and a rectangular wave guide for exciting electric waves in said cylindrical guide at said junction in a plurality of modes when said rectangular wave guide is energized,

one of which modes has its electrostatic lines of force disposed in a transverse radial manner within said cylindrical guide, a conducting end closure for said cylindrical wave so spaced from the central axis of said rectangular wave guide that waves reflected from said end closure arrive at said junction substantially in phase with respect to waves emanating from said rectangular wave guide and are mutually reinforced with waves of like modes at said junction, means within said cylindrical wave guide for resonantly responding to and reflecting waves of modes other than said one mode, said reflecting means being so positioned between said end closure and said junction that waves reflected by said means arrive at said junction in substantially opposing phase with respect to waves emanating from said rectangular Wave guide and are substantially mutually canceled with waves of like mode at said junction.

, WILLIAM M. PRESTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,180,950 Bowen Nov. 21, 1939 2,197,122 Bowen Apr. 16, 1940 2,223,082 Van Mierlo Nov. 26, 1940 2,267,289 Roosenstein Dec. 23, 1941 2,407,318 Mieher Sept. 10, 1946 2,513,205 Roberts June 27, 1950 r 2,519,750 Ehlers Aug. 22, 1950 

